This invention relates generally to data communications. More particularly, this invention relates to a technique for high bandwidth laser-based data communications.
Solid state fiber lasers have been developed for commercial communication applications. Picosecond pulses (ca 50 ps) are currently being used for fiberoptic based long distance (transpacific) soliton communication. Much shorter pulses ( less than 100 Fs) have been generated with fiber sources, pumped by diode lasers. Such systems have the advantages that they are being tailored for communications, the bandwidth that is used is at an xe2x80x9ceye safexe2x80x9d wavelength, and fast modulation techniques have been or are being developed. There is still work to be done, but the fact that the light is confined to a narrow waveguide helps improve the speed of modulation. The disadvantages of such systems is that they have a lossy transition from fiber to air, they have a larger diffraction angle at longer wavelengths, the minimum size beam at 20 km is about 10 m, there is a low average and peak power (pJ/pulse), and a low repetition rate (1 to 10 MHz) exists.
Solid state Nd vanadate lasers (Nd:YVOxe2x80x944) could also be used in communication systems. These diode pumped lasers produce a train of pulses of about 5 ps duration (which could be compressed externally to the laser to 100 fs). The repetition rate is typically 100 MHz. The advantage of systems of this type are high efficiency, high average power-up to 100 W in an infrared beam, 20 W in the green, which can be frequency-tripled to a wavelength of 355 nm. The typical pulse energy in the green is 0.2 uJ/pulse. Pump diodes for this type of laser have a long lifetime; they are actively under development and they are relatively cost effective. In addition, these systems can provide diffraction limited beams in the green and UV. For the green, the minimum spot size at 20 km is about 2 m. For the V, the minimum spot size at 20 km is about 1.5 m.
Solid state Cr:LiSAF Lasers (also Cr:LiSGAF and Cr:LiCAF) are another option for communication systems. Tunable pulse generation in the range of 820 nm to 880 nm has been demonstrated. Pulse duration as short as 20 fs has been obtained, but at very low average power. A maximum average power has been demonstrated at 1.1 W, for continuous operation, while short pulse operation is only 0.5 W. Advantageously, these systems have shorter pulses directly out of the laser, without compression. In addition, they have a shorter wavelength. The disadvantages of these systems is that they have less efficient pump diodes and less average power.
Existing laser-based communication systems rely upon light modulation through electronic control techniques. Thus, the speeds of such systems are inherently limited to the speeds of the electronic control systems. It would be highly desirable to to eliminate the speed limitation of electronic control systems. In particular, it would be highly advantageous to substitute an electronic control system with an optical control system to enhance the performance of a laser-based communication system.
Existing laser-based communication systems use fiber optic channels to communicate information. It would be desirable to reduce the expense and complexity of a laser-based communication system by eliminating the fiber optic channel. The absence of a fiber optic channel would allow more flexibility in developing communication system infrastructures.
A laser communication system includes a first laser to generate a laser signal with femtosecond pulses. A first grating spectrally disperses the femtosecond pulses of the laser signal. A modulator converts the femtosecond pulses of the laser signal into a string of pulses representative of coded words. A second grating spectrally recombines the coded words of the laser signal. A first telescope launches the laser signal. A second telescope receives the laser signal. A second laser generates a set of reference pulses. A non-linear crystal combines the set of reference pulses and the laser signal so as to create an output signal only when the laser signal and the reference pulses temporally coincide. A detector senses and records the output. The first laser also generates a synchronization signal. The second laser is synchronized by the synchronization signal.